Practical Digital Pre -Distortion Techniques for PA...

Practical Digital Pre-Distortion Techniques for PA Linearization

in 3GPP LTE

Copyright Agilent Technologies 2010SystemVue DPD

Jinbiao XUMay 26, 20101

Jinbiao XuAgilent Technologies

Master System Engineer

Agenda

• Digital PreDistortion----Principle• Crest Factor Reduction• Digital PreDistortion Simulation• Digital PreDistortion Hardware Verification



Digital Pre-Distortion----- Principle

θo-pd = k θout

Input Power

Output Phase

Pi Pi-pd

Desired Output Linear Output



Digital Pre-Distortion----- Principle

4Copyright Agilent Technologies 2010

SystemVue DPDJinbiao XU

May 26, 2010

The DPD-PA cascade attempts to combine two nonlinear systems into one linear result which allows the PA to operate closer to saturation.The objective of digital predistorter is to have , where C is a constant.)()( tCxty ≈

The most important step is to extract PA nonlinear behavior accurately and efficiently.

Memory Polynomial Algorithm• As the signal (such as 3GPP LTE) bandwidth gets wider, power amplifiers

begin to exhibit memory effects. Memoryless (LUT) pre-distortion can achieve only very limited linearization performance.

• Volterra series is a general nonlinear model with memory. It is unattractive for practical applications because of its large number of coefficients.

• Memory polynomial reduces Volterra’s model complexity. It is interpreted as a special case of a generalized Hammerstein model. Its equation is as follows:

K is Nonlinearity order and Q is Memory order

Copyright Agilent Technologies 20105


May 26, 2010

∑∑= =

−−−=K

k

Q

q

kkq qnyqnyanz

1 0

1)()()(

Signal Training to derive the Memory Polynomial

1. Pre-distorter training: Nonlinear coefficients are extracted from the PA input and PA output waveforms (ie – on real physical behavior)

2. Copy of PA : The DPD model accurately captures the nonlinearity with memory effects


zUUUa HH 1)(ˆ −=

[ ]TNzzzz )1(,),1(),0( −= [ ]KQQK uuuuU ,,,,,, 1010 =

[ ]Tkqkqkqkq Nuuuu )1(,),1(),0( −=

1)()()(−−−

=k

kq Gqny

Gqnynu

Memory Polynomial Coefficients

[ ]TKQQK aaaaa ˆ,,ˆ,,ˆ,,ˆˆ 1010 =


May 26, 2010

• Spectrally efficient wideband RF signals may have PAPR >13dB.• CFR preconditions the signal to reduce signal peaks without significant

signal distortion• CFR allows the PA to operate more efficiently – it is not a linearization

technique• CFR supplements DPD and improves DPD effectiveness• Without CFR and DPD, a basestation PA must operate at significant

back-off from saturated power to maintain linearity. The back-off reduces efficiency

Benefits of CFR1. PAs can operate closer to saturation, for improved efficiency (PAE).2. Output signal still complies with spectral mask and EVM specifications

Crest Factor Reduction (CFR) Concepts



May 26, 2010

Crest Factor Reduction (CFR) Concepts

If you can reduce the Peak-to-Average Ratio of the signal, then for a given amplitude Peak, you can raise the Average power (up & to the right, above) with no loss in signal quality.

Thus, CFR enables higher PA efficiency by reducing the back-off, often by 6dB



May 26, 2010

Crest Factor Reduction for Multiple-Carrier Signals• Multiple-Carrier Signals (such as GSM, WCDMA, WiMAX) already have high

PAPR. • In the future, they will also include multiple waveforms (ie - LTE with 3G WCDMA).• Therefore CFR will increase in importance for Multi-Carrier PA (MCPA)

linearization.


CFR algorithm for multiple carrier signals• PW (Peak Windowing)-CFR• NS (Noise-Shaping) -CFR• PI (Pulse Injection)-CFR• PC (Peak Cancellation)-CFR


May 26, 2010

CFR for 3GPP LTE DL OFDM Signal• Controls EVM and band limits in the frequency domain.

• Constrains constellation errors, to avoid bit errors.• Constrains the degradation on individual sub-carriers.

• Allows QPSK sub-carriers to be degraded more than 64 QAM sub-carriers.

• Does not degrade reference signals, P-SS and S-SS.• All control channels (PDCCH, PBCH, PCFICH and PHICH) adopts

QPSK threshold.



May 26, 2010

LTE CFR (Crest Factor Reduction)

LTE Downlink 10MHz, Sampling Rate 61.44MHz,QPSK,EVM threshold 10%

Simulation Results



DPD Simulation Workspace

Step 1 is to Generate Waveform for DPD

Step 3 is for DUT Model Extraction

Step 4 is for DPD Response

Compared with hardware verification tool, simulation tool does not include Step 2 and Step 5.

Hardware verification toll will be introduced later.



LTE DPD simulation for a memoryless nonlinear PA

EVM (dB)

ACLR (dB)



LTE DPD simulation for a nonlinear PA with memory

EVM (dB)

ACLR (dB)



DPD Hardware Verification FlowchartCreate DPD Stimulus

Capture DUT Response

DUT Model Extraction

DPD Response

Verify DPD Response

DPD HW Flowchart consists of 5 steps:• Step 1 (Create DPD Stimulus) is to

download waveform (LTE or User defined) into ESG/MXG.

• Step 2 (Capture DUT Response) is to capture both waveforms before power amplifier and after power amplifier from PSA/MXA/PXA by using VSA89600 software.

• Step 3 (DUT Model Extraction) is to extract PA nonlinear coefficients based on both captured PA input and PA output waveforms and then to verify DPD by using PA nonlinear coefficients.

• Step 4 (DPD Response) is to download the waveform (LTE or User Defined) after pre-distorter (by using PA nonlinear coefficient from Step 3) into ESG/MXG, this real signal passes through the PA DUT, capture PA output waveform from PSA/MXA/PXA by using VSA89600 software.

• Step 5 (Verify DPD Response) is to show the performance improvement after DPD.



DPD Hardware Verification Workspace Structure



DPD Hardware Verification Platform

10MHz Reference

10MHz Reference

External Trigger

External Trigger

Attenuator

1. PA input signal capture

2. PA output signal capture

Signal source:LTE 10MHz

Agilent MXG/ESG PSA/MXA/PXA

PSA/MXA/PXAMXG/ESG



DPD Hardware Verification – LTE (Step 1)Step 1: Create Stimulus The CFR must be enable in LTE

source.LTE paramters (such as bandwidth, Resource Block allocation and etc) can be set.

The download waveform transmit power, length also can be set.



Step 2: Capture DUT Response

DPD Hardware Verification – LTE (Step 2)

Firstly, connect the ESG directly with the PSA/PXA and click the “Capture Waveform” button in the “Capture PA Input” panel in the GUI. The captured signal is the input of the PA DUT.

Then, connect the ESG with the DUT, and then connect the DUT with the PSA/PXA and click the “Capture Waveform” button in the “Capture PA Output” panel in the GUI. The captured signal is the output of the PA DUT.

These I/Q files are stored for further usage.



Step 3: DUT Model Extraction


This step is to extract PA nonlinear coefficient from the PA input and PA output waveform and get the coefficients of the DPD model.

DPD Verification AM-AM



DPD Hardware Verification – LTE (Step 4)Step 4: DUT Response This step is to apply the DPD model extracted in

Step 3. The generated LTE downlink signal is firstly pre-distorted by the extracted model, and then downloaded into the ESG.




SpectrumEVMACLR

Step 5: Verify DUT Response

This step is to verify the performances of the DPD (including spectrums of the DUT output signal w/ and w/o DPD, EVM and ACLR).

EVM (dB)

ACLR (dB)



EVM (dB)

ACLR (dB)

Hardware Verification Results of Doherty PA



1. Lei Ding, Zhou G.T., Morgan D.R., Zhengxiang Ma, Kenney J.S., Jaehyeong Kim, Giardina C.R., “A robust digital baseband predistorter constructed using memory polynomials”, Communications, IEEE Transactions on, Jan. 2004, Volume: 52, Issue:1, page 159-165.

2. Lei Ding, “Digital Predistortion of Power Amplifiers for Wireless Applications”, PhD Thesis, March 2004.3. Roland Sperlich, “Adaptive Power Amplifier Linearization by Digital Pre-Distortion with Narrowband Feedback using

Genetic Algorithms”, PhD Thesis, 2005.4. Helaoui, M. Boumaiza, S. Ghazel, A. Ghannouchi, F.M., “Power and efficiency enhancement of 3G multicarrier

amplifiers using digital signal processing with experimental validation”, Microwave Theory and Techniques, IEEE Transactions on, June 2006, Volume: 54, Issue: 4, Part 1, page 1396-1404.

5. H. A.Suraweera, K. R. Panta, M. Feramez and J. Armstrong, “OFDM peak-to-average power reduction scheme with spectral masking,” Proc. Symp. on Communication Systems, Networks and Digital Signal Processing, pp.164-167, July 2004.

6. Zhao, Chunming; Baxley, Robert J.; Zhou, G. Tong; Boppana, Deepak; Kenney, J. Stevenson, “Constrained Clipping for Crest Factor Reduction in Multiple-user OFDM”, Radio and Wireless Symposium, 2007 IEEE Volume , Issue , 9-11 Jan. 2007 Page(s):341- 344.

7. Olli Vaananen, “Digital Modulators with Crest Factor Reduction Techniques”, PhD Thesis, 2006 8. Boumaiza, et a, “On the RF/DSP Design for Efficiency of OFDM Transmitters” , IEEE Transactions on

Microwave Theory and Techniques, Vol. 53, No. 7, July 2005, pp 2355-2361.9. Boumaiza, Slim, “Advanced Memory Polynomial Linearization Techniques,” IMS2009 Workshop WMC

(Boston, MA), June 2009.10. Amplifier Pre-Distortion Linearization and Modeling Using X-Parameters, Agilent EEsof EDA

References



May 26, 2010

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